From Halopedia, the Halo wiki
The Z-12 was initially created in 2319, with the first Z-12 prototypes quickly making a name for themselves with AMG's well-crafted advertising campaign and exceptional engineering team that showcased the prototype’s impressive feature set and unparalleled functionality in the unpopulated outlands of both Luna and Mars. It was lauded as the vehicle that could go anywhere and do anything.
During the first eight months of the prototype’s existence, AMG Transport Dynamics received over three dozen exclusivity contracts, the largest and most lucrative being with the Colonial Military Administration (CMA). By 2321, AMG Transport Dynamics had secured a deal with the CMA to meet nearly all of their land-based transportation needs. It was at this time that the Z-12 became the M12 Force Application Vehicle (FAV) with the addition of a pneumatically powered swivel mount and armored body panels.